1. Field of the Invention
The present invention relates to Li ion nonaqueous electrolyte secondary battery.
2. Description of the Related Art
A nonaqueous electrolyte secondary battery that exerts charging and discharging through migration of Li ions between a negative electrode and a positive electrode is being actively developed as a high energy density battery.
A Li ion nonaqueous electrolyte secondary battery using lithium cobaltate as a positive electrode active substance and carbon as a negative electrode active substance is subjected to practical use, and the battery voltage thereof is about 3.8 V at most. As one measure for further improving the energy density, it is considered to increase the battery voltage.
There has been proposed such an attempt of using LiMn1.5Ni0.5O4 having a positive electrode potential in a full charged state of about from 4.9 to 5.0 V (which is a potential with respect to metallic lithium, hereinafter, referred to as “vs. Li”) (see JP-A-2000-515672). A combination of the positive electrode and a negative electrode using carbon having a negative electrode potential in a full charged state of about 0.1 V (vs. Li) as a negative electrode active substance realizes a nonaqueous electrolyte secondary battery having a battery voltage fairly close to 5 V.
For example, there has been disclosed a nonaqueous electrolyte secondary battery having the aforementioned positive electrode, a negative electrode using a Li—Sn alloy and carbon as a negative electrode active substance, and a nonaqueous solvent containing a mixture of 75% by volume or more of a linear ester, such as diethyl carbonate, and a cyclic ester, such as ethylene carbonate (see JP-A-2002-352797).
As a result of earnest investigations made by the inventors, it has been found as follows.
The aforementioned nonaqueous electrolyte secondary battery suffers oxidation decomposition of the nonaqueous electrolyte in the vicinity of the positive electrode in a high potential state. The product formed through the oxidation decomposition is accumulated or deposited on the negative electrode having a lower potential to produce a parasitic resistance. The nonaqueous electrolyte secondary battery is deteriorated in cycle capability, particularly high temperature cycle capability, due to the phenomenon.